1. Field of the Invention
The present invention relates to a titanium-based catalyst, a method of preparing the same, and a method of using the same in an ethylene polymerization process.
2. Description of the Related Art
Titanium-based catalyst compositions have been known for use in polymerizing ethylene. For example, European Patent Publication No. 0 120 503 discloses a titanium-based catalyst that is used in producing low density ethylene copolymers. The catalyst composition disclosed therein is formed by impregnating, or mixing, a composition of formula (A) into an aluminum compound-treated silica, followed by activation with a co-catalyst of the formula (B): EQU Mg.sub.m Ti(OR).sub.n X.sub.p [ED].sub.q (A) EQU Al(R").sub.d X'.sub.e H.sub.f (B)
wherein R is an aliphatic or aromatic hydrocarbon radical containing 1-14 carbon atoms, or COR', wherein R' is also an aliphatic or aromatic hydrocarbon radical containing 1-14 carbon atoms,
X is selected from the groups consisting of Cl, Br and I or mixtures thereof,
ED is an electron donor compound,
m is 0.5-56, preferably 1.5-5,
n is 0, 1, or 2,
p is 2-116, preferably 6-14,
q is 2-85, preferably 3-10,
X' is Cl or OR",
R" is an aliphatic or aromatic hydrocarbon radical containing 1-14 carbon atoms, wherein each R" substituent may be the same or different,
e is 0-1.5,
f is 0 or 1, and
d+e+f is 3.
The catalyst thus obtained is shown to be useful in the polymerization of ethylene copolymers having a density of below 0.91 g/cm.sup.3 and a melt flow ratio of 37-40. However, it is difficult to increase the density of the copolymer to greater than 0.91 g/cm.sup.3. Indeed, the ethylene copolymer produced according to this process is generally considered a "very low density polyethylene." Therefore, the copolymer produced using this catalyst has poor strength, such as for film processing.
Another titanium-based catalyst composition is disclosed in U.S. Pat. No. 4,379,758. Here, after impregnating and/or mixing a precursor of the above formula (A) into a carrier, the precursor is reacted with a boron halide compound having the formula (C): EQU BR.sub.c X'.sub.3-c (C)
wherein R is an aliphatic or aromatic hydrocarbon radical containing 1-14 carbon atoms or OR', wherein R' is also an aliphatic or aromatic hydrocarbon radical containing 1-14 carbon atoms,
X' is selected from the group consisting of Cl, Br, and mixtures thereof, and
c is 0 or 1 when R is an aliphatic aromatic hydrocarbon and 0, 1 or 2 when R is OR'.
The boron halide compounds can be used individually or in combination, and include BCl.sub.3, BBr.sub.3, B(C.sub.2 H.sub.5)Cl.sub.2, and B(OC.sub.2 H.sub.5)Cl.sub.2, most preferably BCl.sub.3.
After treatment with the boron halide compound, the composition is then activated with a co-catalyst of formula (B). The catalyst is suitable for use in the polymerization of ethylene and forms a so called "high density polyethylene."
However, the melt flow ratio of the high density polyethylene produced using the catalyst in U.S. Pat. No. 4,379,758 is unsatisfactorily low and thus, affords poor productivity, i.e. difficult extrusion, etc.